Frequency sweep for a high-pass filter???

We had a basic resistive-capacitive high-pass filter. In the manual frequency sweep the output voltage across the resistor increased with frequency as it was supposed to until we reached 1MHz, then the voltage dropped. What could cause this? Would the capacitor beaking down cause this? We are working with really old capacitors.

We had a basic resistive-capacitive high-pass filter. In the manual frequency sweep the output voltage across the resistor increased with frequency as it was supposed to until we reached 1MHz, then the voltage dropped. What could cause this? Would the capacitor beaking down cause this? We are working with really old capacitors.

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Capacitors are not perfect as well as resistors and wires. Your capacitors may be broken but what i suspect is that after 1 MHz the stray inductance of the capacitors leads and the inductance of your wires (if they are long) cause the voltage to drop because the stray inductance in series with the capacitor form a band-pass filter.

"Ideal" capacitors consist of only capacitance - but they exist only in simulations.
"Real" capacitors, along with capacitance, have relatively small parasitic values of resistance and inductance. The higher the frequency, the more apparent these parasitics become. See the attached.

All electronic components have parasitic values besides their main attribute. Even resistors have parasitic inductance and capacitance.